The present invention relates to a system, based on the use of immature gametic cells, for influencing the fertility of plant material that is phenotypically sterile by virtue of a manipulation such as interspecific or intergeneric sexual hybridization or somatic fusion, somaclonal selection, genetic transformation or mutagenesis.
As advances are made in different methods for producing useful variants from existing plant material, it has become evident that the effective sterility which is characteristic of many such variants presents a major hurdle to their widespread use in breeding programs. It is well known, for example, that sexual crossability between phylogenetically distant plant types is generally limited by incompatibility resulting from disturbed pollen/stigma and pollen-tube/style interactions, from failure of zygote development, and from embryo or endosperm disruption.
Although embryo rescue may permit one to obtain variants from an interspecific or intergeneric sexual crossing, such variants are nearly always virtually sterile. For example, Mohapatra and Bajaj, Euphytica 36: 321-26 (1987), attempted the interspecific hybridization via embryo rescue of an oil-yielding Brassica species, B. juncea, with white mustard (B. hirta), which is resistant to leaf blight, but the F.sub.1 plants displayed large morphological diversity, including abnormal, aneuploid and sterile flowers, and markedly reduced fertility.
There are also numerous reports in the literature that concern the sterility or near-sterility of many somatic hybrids. A frequent result of protoplast fusions aimed at producing interspecific and intergeneric hybrids has been the development of sterile or near-sterile plants Thus, Hoffman and Adaihi, Planta 153: 586-93 (1981) describe a sterile intergeneric somatic hybrid produced by fusion of Arabidopsis sp. and Brassica sp. protoplasts. In IAPTC Newsletter, No. 38 (October, 1982), at pages 6-12, Harms et al attribute results such as these to various mechanisms of somatic incompatibility, including chromosome rearrangement, nuclear-cytoplasmic interaction and interorganelle competition.
By the same token, the production of transgenic plants, for example, by Agrobacterium-mediated transformation or direct DNA transfer, can be accompanied by modifications of the donor DNA (deletions, partial duplications, point mutations). See Czernilofsky et al, DNA 5: 473-82 (1986). Modifications of this sort may explain the high incidence of sterility among transgenic plants, for example, as reported by Chyi et al, Mol. Gen. Genet. 204: 64-69 (1986). Sterility problems are also prevalent in applications of culturing techniques to exploit mutational and somaclonal variation. Thus, Evans and Sharp, Bio/Technology 4: 528-32 (1986), and D'Amato, CRC Critical Rev. Plant Sci. 3: 73-112 (1985), note the widespread incidence of altered chromosome number and other chromosomal aberrations among variant plants regenerated from cell culture.
The conventional approach to overcoming phenotypic sterility in plant variants has been to attempt a sexual cross between the variant and another plant that may provide a fertility-restoring genetic contribution to some of the progeny. This approach is limited by its requirement that there be no intractable sexual-incompatibility barriers to the proposed cross, and that the progeny of the cross are viable. The conventional approach is also limited to the extent that the contemplated sexual cross employs pollen from the variant, even when production of normal pollen by the variant may be only a fraction of normal levels. Moreover, use of pollen from the variant means that the latter's cytoplasmic traits, which are in general inherited solely maternally, will probably be lost to any fertile progeny.